The present invention relates to an acoustic surface wave device which serves as a bandpass filter, a delay line or the like.
In general, conversion from acoustic surface waves propagating across a piezoelectric material, such as LiNbO.sub.3 or LiTaO.sub.3, to electrical energy and vice versa is carried out by a transducer which is conventionally composed of a pair of separated, interdigitated comb-shaped electrodes formed on the surface of the piezoelectric material.
One conventional acoustic surface device comprises a piezoelectric substrate having a planer surface for propagation of acoustic surface waves and a pair of input and output transducers formed on the surface of the substrate. When an electrical signal is applied to the input transducer, the electrical signal is converted into acoustic surface waves which are propagated to the output transducer. As a result, the acoustic surface waves are again converted into electrical signals by the output transducer. There are mainly three kinds of such acoustic waves, which reach the output transducer, as follows:
(1) desired acoustic surface waves which are launched by the input transducer and are propagated on the surface of the substrate to reach the output transducer;
(2) bulk waves which are launched by the input transducer and are propagated through the body of the substrate to reach the output transducer, and;
(3) surface reflections which are launched by the input transducer and are reflected by the output transducer and, in addition, are reflected by the input transducer to reach the output transducer.
A surface reflection as mentioned above is called a triple transit echo (hereinafter referred to as a TTE), since the reflections travel three times between the input and output transducers. Both of the bulk waves and the TTE reduce the bandpass characteristics, since their transit times are different from that of the desired acoustic surface waves.
Another conventional acoustic surface wave device comprises: a piezoelectric substrate having a planer surface for propagation of acoustic surface waves; an input transducer formed on the surface of the substrate for converting electrical energy into the acoustic surface waves; an output transducer formed on the surface of the substrate and located diagonally with respect to the input transducer, for converting the acoustic surface waves into electrical energy; and a multistrip coupler (hereinafter referred to as an MSC), including a plurality of parallel and equally spaced conductors, formed on the surface of the substrate and interposed between the input and output transducers so as to be substantially orthogonal to the propagation direction of the acoustic surface waves launched by the input transducer. The MSC transfers the propagation direction of acoustic surface waves from one track to another track. In other words, the MSC serves as an acoustic surface wave path changer only for the acoustic surface waves, not for bulk waves which travel through the body of the substrate. Therefore, the bulk waves can be prevented from reaching the output transducer. The MSC which is used for such a purpose has been designed by paying attention to wide bandpass characteristics. The characteristics of the input and output transducers are determined by weighting the cross-length of bamboo-blind shaped electrodes, i.e., by weighting electrodes.
On the other hand, the use of double electrode structures in the electrode of the transducer is known to provide a useful improvement in the level of TTE (Ref. ELECTRONICS LETTERS, Vol. 10, No. 24). In the double electrode structures, the spacing of electrodes is .lambda./4 where .lambda. is the wavelength of the desired acoustic surface waves. Therefore, the difference in phase between two reflections from two adjacent electrodes is .pi. and, accordingly, all reflections are diminished. However, in the acoustic surface wave device using transducers having bamboo-blind shaped electrodes, spurious signals, in addition the desired response in the proximity of the frequency f.sub.o, are present in the proximity of the frequency 3f.sub.o where f.sub.o is the frequency of the desired acoustic surface waves, which also result in reduction of the response characteristics of the stopband frequencies of the device.